1. Field of the Invention
The present invention relates to a feed control apparatus for controlling rapid and machining feed movements of a tool or workpiece carriage by changing the rotational speed of a servomotor drivingly connected to the carriage. Particularly, the invention relates to such a feed control apparatus suitable for use in grinding machines.
2. Description of the Prior Art
In a grinding machine as disclosed in U.S. Pat. No. 4,115,958 to G. E. Englander et al., a carriage rotatably supporting a grinding wheel is moved by a single servomotor at a rapid non-grinding feed rate until a point just ahead of a programmed fast feed end point is reached and then at a slow grinding feed rate until a workpiece is finished to a predetermined size. If rapid feed movement of the carriage is continued beyond the programmed fast feed end point due to malfunction of the circuit which controls rotation of the servomotor, the grinding wheel could be brought into engagement with the workpiece at the rapid feed rate, and this results in breakage of the grinding wheel, fault of the workpiece, and the like.
In order to prevent the grinding wheel from engaging the workpiece at the rapid feed rate, measures have been taken in the grinding machine to shut down the same in response to a signal from a sensor in the event that the current required by a motor to rotate the grinding wheel exceeds a predetermined relatively low idle current during rapid feed movement. However, since it is after engagement of the grinding wheel with the workpiece that the measures begin their functions to shut down the grinding machine, there is no substantial time until the grinding wheel begins to be broken. It is therefore technically difficult to use such measures in the case where the rapid feed rate of the grinding wheel is so high as to reach 5 m/sec.